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Mozilla is working on patching its Firefox browser after a hacker posted details of a flaw that could let criminals run unauthorized software on a victim's machine.

The flaw lies in Firefox's URL handler component, which was the source of another bug, disclosed Tuesday by Mozilla.

This second flaw was disclosed Tuesday by Billy Rios and Nathan McFeters, security consultants with Verisign and Ernst & Young respectively.

Like the first flaw, this one could be exploited by attackers to launch programs on the victim's PC without authorization, said Tyler Reguly, a security research engineer at nCircle Network Security. "They're both related to the URL handling process," he said "It's just different errors within that handling process."

Even though the code posted by Rios and McFeters can only be used to launch software that is already installed on a victim's PC, it could be very dangerous if used by criminals, Reguly said. "It's still letting you run any program that exists on the user's computer," he said. "You can make it do some fairly bad things. For example, having it use command-line FTP to download a malicious file off a server somewhere and then execute that file."

A victim would have to be tricked into clicking on a malicious link for the attack to work, Reguly said.

Mozilla's security chief Window Snyder, said that her team is working to verify and fix this latest flaw.

Firefox's URL handler has been a headache for Mozilla ever since security researcher Thor Larholm showed that the way Internet Explorer (IE) and Firefox interact with each other could be exploited to launch software on a user's machine without authorization. To make the attack work, IE would load malformed data from a Web site, and would then send it to Firefox, which would launch the unauthorized software

Microsoft and Mozilla disagreed about who was at fault, however. Snyder initially said that the attack wouldn't work on Firefox alone and that Microsoft should change the way IE passes malformed data to other programs. Microsoft said that the problem lay with Firefox.

While disclosing details on the first URL handler bug on Tuesday, Snyder admitted that she was wrong. "We thought this was just a problem with IE. It turns out, it is a problem with Firefox as well," she said. "We should have caught this scenario."

Mozilla is planning to fix this issue in the upcoming 2.0.0.6 release of its browser. Snyder did not say when the Billy Rios bug would be patched.

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